Submatch Extraction

ABSTRACT

A method for submatch extraction may include receiving an input string, receiving a regular expression, and converting the regular expression with capturing groups into a plurality of finite automata to extract submatches. The method further includes using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string, and using states of the first automaton in a second automaton to extract the submatches.

BACKGROUND

Regular expressions provide a concise and formal way of describing a set of strings over an alphabet. Given a regular expression and a string, the regular expression matches the string if the string belongs to the set described by the regular expression. Regular expression matching may be used, for example, by command shells, programming languages, text editors, and search engines to search for text within a document. Known techniques for regular expression matching can have long worst-case matching times.

BRIEF DESCRIPTION OF DRAWINGS

Features of the present disclosure are illustrated by way of example and not limited in the following figure(s), in which like numerals indicate like elements, in which:

FIG. 1 illustrates an architecture of a submatch extraction system, according to an example of the present disclosure;

FIG. 2 illustrates an architecture of a pattern creation module of the submatch extraction system, according to an example of the present disclosure;

FIG. 3 illustrates rules for the construction of an automaton M₁, according to an example of the present disclosure;

FIGS. 4A and 4B respectively illustrate a deterministic finite automaton (DFA) M₁ for ((a)*|b)(ab|b), and a nondeterministic finite automaton (NFA) M₂ for the same regular expression, according to an example of the present disclosure;

FIG. 5 illustrates an automaton M₃ for the regular expression ((a)*|b)(ab|b), where v={16}, w={13, 14}, x={8}, y={5}, and z={5, 12}, according to an example of the present disclosure;

FIG. 6 illustrates relationships among q, r, P, p, and τ in the construction of the automaton M₄, according to an example of the present disclosure;

FIGS. 7A and 7B illustrate the automaton M₄ for the regular expression ((a)*|b)(ab|b), showing inputs from Q₃ and outputs in T*, respectively, and FIG. 7C illustrates a simplified diagram of an input string, automata M₃ and M₄, and boundaries, according to an example of the present disclosure;

FIG. 8 illustrates a method for submatch extraction, according to an example of the present disclosure;

FIG. 9 illustrates a further detailed method for submatch extraction, according to an example of the present disclosure; and

FIG. 10 illustrates a computer system, according to an example of the present disclosure.

DETAILED DESCRIPTION

For simplicity and illustrative purposes, the present disclosure is described by referring mainly to examples thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be readily apparent however, that the present disclosure may be practiced without limitation to these specific details. In other instances, some methods and structures have not been described in detail so as not to unnecessarily obscure the present disclosure.

Throughout the present disclosure, the terms “a” and “an” are intended to denote at least one of a particular element. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on.

1. Overview

Regular expressions are a formal way to describe a set of strings over an alphabet. Regular expression matching is the process of determining whether a given string (for example, a string of text in a document) matches a given regular expression, that is, whether it is in the set of strings that the regular expression describes. Given a string that matches a regular expression, submatch extraction is a process of extracting substrings corresponding to specified subexpressions known as capturing groups. This feature provides for regular expressions to be used as parsers, where the submatches correspond to parsed substrings of interest. For example, the regular expression (.*)=(.*) may be used to parse key-value pairs, where the parentheses are used to indicate the capturing groups.

A submatch extraction system and a method for extracting submatches from a string that matches a regular expression are described herein. The system and method provide reduced submatch extraction times in the worst case. The submatch extraction system may include an input module to receive a regular expression. The regular expression may be used to create a pattern by a pattern creation module. In order to create the pattern, automata M₁, M₂, M₃ and M₄ may be respectively created by automata generation modules. An automaton is defined as an abstract machine that can be in one of a finite number of states and includes rules for traversing the states. The automata may be stored in the system as machine readable instructions. A comparison module may receive input strings, and match the input strings to the regular expression. If an input string does not match the regular expression, submatches are not extracted. However, if an input string matches the regular expression, the pattern created may be used to extract submatches by an output module. In this manner, the regular expression may be compiled by the automata generation modules, and then matched to many different input strings to extract submatches.

In an example, the submatch extraction system may generally include a memory storing a module comprising machine readable instructions to receive an input string, receive a regular expression, and convert the regular expression into a plurality of finite automata to extract submatches. The extracting may include using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string, and using states of the first automaton as input to a second automaton to extract the submatches. The system may include a processor to implement the module.

In an example, the method for submatch extraction may include receiving an input string, receiving a regular expression, and converting the regular expression with capturing groups into a plurality of finite automata to extract submatches. The method may further include using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string, and using states of the first automaton in a second automaton to extract the submatches.

A non-transitory computer readable medium may have stored thereon machine readable instructions for submatch extraction is also described. The machine readable instructions that when executed may cause a computer system to receive an input string, receive a regular expression, and convert the regular expression into a plurality of finite automata to extract submatches. The extracting includes using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string, and using states of the first automaton in a second automaton to extract the submatches.

For regular expressions, the reluctant closure operator, denoted *?, is a variant of the standard greedy closure operator for regular expressions, denoted *, with different submatching behavior. Where other rules do not apply, shorter submatches to a subexpression E*? take priority over longer ones, whereas for E* the reverse is true.

For example, consider matching the string a=b=c first against the regular expression (.*?)=(.*), where the meta-character ‘.’ matches any character in the alphabet, and then against the regular expression (.*)=(.*?). In the first case, the two capturing groups (which are delimited by parentheses) should match a and b=c, respectively, while in the second case the respective submatches should be a=b and c. If the two capturing groups are both greedy or both reluctant, then matching behavior could in principle be governed by a standard related to precedence, although no suitable standard exists.

With regard to parsing, parsing using regular expressions may be used as a building block for security applications such as security information and event management (SIEM) systems. SIEM systems perform real-time analysis of event logs and security alerts generated by hardware and software systems in an enterprise network. Since each source generates its logs in a different format, a SIEM system may use submatch extraction to parse common fields, such as, for example, device name and attack source from the logs. In such a setting, a relatively small number of regular expressions, which are known in advance, may be matched to a large number of input strings in real time. In this regard, the submatch extraction system and method provide for efficient submatch extraction when matching a string to a regular expression that may contain reluctant operators, where the expression may be compiled in advance into a form that will speed up matching and submatching.

For the example of a submatch extraction system whose construction is described in detail herein, the syntax of regular expressions with capturing groups and reluctant closure on an fixed finite alphabet Σ, for example the standard ASCII set of characters, is:

E::=ε∪α∪EE ΠE|E ∪E* ∪E* ^(?)∪(E)∪[E]  Equation (1)

For Equation (1), a stands for an element of Σ, and ε is the empty string. The square brackets [,] are used to group terms in a regular expression that are not capturing groups. The parentheses (,) are reserved for marking capturing groups. Grouping terms is not explicitly needed when the order of operations is clear. Specifically, (,) and [,] have a higher priority than * and *?, which have a higher priority than concatenation, which has a higher priority than |. The submatch extraction system may use this syntax. Other examples of the submatch extraction system may perform submatch extraction for regular expressions written in a syntax that uses different notation to denote one or more of the operators introduced in Equation (1); or that does not include either or both of the operators * or *? in Equation (1); or that includes additional operators, such as, for example, special character codes, character classes, boundary matchers, quotation, etc.

Indices may be used to distinguish the capturing groups within a regular expression. Given a regular expression E containing c capturing groups marked by parentheses, indices 1, 2, . . . c may be assigned to each capturing group in the order of their left parentheses as E is read from left to right. The notation idx(E) may be used to refer to the resulting indexed regular expression. For example, if E=((a)*|b)(ab|b) then idx(E)=((a)*₂|lb)₁(ab|b)₃.

If X, Y are sets of strings, XY is used to denote {xy:xεX,yεy}, and X|Y to denote X∪Y. If β is a string and B a set of symbols, β|_(B) denotes the string in B* obtained by deleting from β all elements that are not in B. A set of symbols T={S_(t), E_(t):1≦t≦c} are introduced and may be referred to as tags. The tags may be used to encode the start and end of capturing groups. The language L(F) for an indexed regular expression F=idx(E), where E is a regular expression written in the syntax given by Equation (1), is a subset of (Σ∪T)*, defined by L(ε)={ε}, L(a)={a}, L(F₁F₂)=L(F₁)L(F₂), L(F₁|F₂)=L(F₁)∪L(F₂), L(F*)=L(F*^(?))=L(F)*, L([F])=L(F), and L((F)_(t))={S_(t):αεL(F)}, where ( )_(t) denotes a capturing group with index t. There are standard ways to generalize this definition to other commonly-used regular expression operators, so that it can be applied to cases where the regular expression E is written in a commonly-used regular expression syntax different from the syntax given in Equation (1).

A valid assignment of submatches for regular expression E with capturing groups indexed by {1,2, . . . c} and input string a is a map sub: {1, 2, . . . c}→Σ*∪{NULL} such that there exists βεL(E) satisfying the following three conditions:

-   β|_(Σ)=α; -   (ii) if S_(t) occurs in β then sub(t)=β_(t)|_(Σ), where β_(t) is the     substring of β between the last occurrence of S_(t) and the last     occurrence of E_(t); and -   (iii) if S_(t) does not occur in β then sub(t)=NULL.

If αεΣ*, a matches E if and only if α=β|_(Σ) for some βεE L(E). For a regular expression without capturing groups, this coincides with the standard definition of the set of strings matching the expression. By definition, if there is a valid assignment of submatches for E and α, then a matches E. It may be proved by structural induction on E that the converse is also true, that is, whenever E matches α, there is at least one valid assignment of submatches for E and α. The submatch extraction system may take as input a regular expression and an input string, and output a valid assignment of submatches to the capturing groups of the regular expression if there is a valid assignment, or report that the string does not match if there is no valid assignment.

The operators (,) and [,] have the same effect as [,] on the set of strings that match a regular expression. The difference is that (,) marks a subexpression whose submatch is to be reported. Similarly, the difference between the operators * and *? is not apparent in the set of valid assignments of submatches, but is apparent in which of these valid assignments is reported.

2. System

FIG. 1 illustrates an architecture of a submatch extraction system 100, according to an example. Referring to FIG. 1, the system 100 may include an input module 101 to receive a regular expression. The regular expression may be used to create a pattern by a pattern creation module 102. The pattern creation module 102 is described in further detail below with reference to FIG. 2. A comparison module 103 may receive input strings, and match the input strings to the regular expression. If an input string does not match the regular expression, submatches are not extracted. However, if an input string matches the regular expression, the pattern created by the pattern creation module 102 may be used to extract submatches by an extraction module 104. Referring to FIG. 2, in order to create the pattern by the pattern creation module 102, automata M₁, M₂, M₃ and M₄ may be respectively created by automata generation modules 105, 106, 107 and 108. Thus, the regular expression may be compiled by the modules 105-108 to create the pattern by the pattern creation module 102. In this manner, the regular expression may be compiled by the modules 105-108, and then matched to many different input strings to extract submatches.

The modules 101-108, and other components of the system 100 may comprise machine readable instructions stored on a computer readable medium. In addition, or alternatively, the modules 101-108, and other components of the system 100 may comprise hardware or a combination of machine readable instructions and hardware.

The components of the system 100 are described in further detail with reference to FIGS. 1-7C.

Referring to FIG. 1, for a regular expression E received by the input module 101, the regular expression E may be fixed and indices may be assigned to each capturing group to form idx(E). In order to create the pattern, the pattern creation module 102 may convert the regular expression E into two deterministic finite automata, denoted M₃ and M₄. The finite automata M₃ and M₄ may be used to match a string. For the real-time operation, the input string may be reversed and consumed by the first automaton M₃, and the states visited as this happens may be journaled. Once all the symbols in the string have been processed, the journaled states may be used in reverse order as input to the second automaton M₄, which is used to determine the start and end locations of each capturing group. These start and end locations may be used by the extraction module 104 to obtain the submatches.

In order to create the pattern, the M₁ and M₂ automata generation modules 105, 106 of the pattern creation module 102 may be used to construct two finite automata, M₁ and M₂. The automaton M₁ is described by the tuple (Q₁, Σ₁, Δ₁, s₁, f₁), where Q₁ is a set of states identified by the integers in the set {1, 2 . . . f}, Σ₁ is the alphabet Σ∪{+, −}∪T, where + and − are two special alphabet characters whose use is described below, Δ₁ is a transition function, s₁=1 is the start state and f₁=f is the unique final state. Δ₁ is built using structural induction on the indexed regular expression, idx(E), following the rules illustrated by the diagrams of FIG. 3. For this example it is assumed that the syntax of the regular expression is that given in Equation (1). In FIG. 3, the initial state of the automaton is marked with > and the final state with a double circle. A dashed arrow with label F or G is used as shorthand for the diagram corresponding to the indexed expression F or G. For example, the automaton M₁ for ((a)*|b)(ab|b) is shown in FIG. 4A. The automata of FIGS. 4A, 4B, 5 and 7A-7C are illustrated as a graphical representation of state machines by way of example and not limitation.

The automaton M₁ uses separate transitions with labels S_(t) and E_(t) to indicate the start and end of a capturing group with index t, in addition to transitions labeled with + and − to indicate submatching priorities.

The automaton M₁ may be considered as a directed graph. If x is any directed path in M₁, Is(x) denotes its label sequence. Let π:Q₁×Q₁→T* be a mapping from a pair of states to a sequence of tags, to be used below in the construction of M₄, defined as follows. For any two states q, pεQ₁, consider a depth-first search of the graph of M₁, beginning at q and searching for p, using only transitions with labels from T∪{+, −}, and such that at any state with outgoing transitions labeled ‘+’ and ‘−’, the search explores all states reachable via the transition labeled ‘+’ before following the transition labeled ‘−’. If this search succeeds in finding successful search path λ(q, p), then π(q, p)=Is(λ(q, p))|_(T) is the sequence of tags along this path. If the search fails, then π(q, p) is undefined. π(p, p) is defined to be the empty string. It can be shown that this description of the search uniquely specifies λ(q, p), if it exists.

The automaton M₁ may be converted into the nondeterministic finite automaton (NFA) M₂, described by the tuple (Q₂, Σ, Δ₂, S₂, F₂), by the M₂ automaton generation module 106. The set Q₂ includes the final state of M₁ together with any state in M₁ that has an outgoing transition labeled with a symbol in Σ, i.e.

Q ₂ ={f}∪{q:∃αεΣ, pεQ ₁,(q, α, p)εΔ₁}  Equation (2)

If p, qεQ₂ and aεΣ, there is a transition (p, a, q)εΔ₂ if and only if there exists a state rεQ₁ such that (p, a, r)εΔ₁ and π(r, q) is defined. S₂ is a set of initial states, corresponding to those states pεQ₂ for which π(1, p) is defined. For example, the automaton M₂ for ((a)*|b)(ab|b) is shown in FIG. 4B.

The automaton M₂ may be converted into the deterministic finite automaton (DFA) M₃ by the M₃ automaton generation module 107, and is specified by the tuple (Q₃, Σ, Δ₃, s₃, F₃). The construction of M₃ from M₂ is a standard powerset construction of a DFA from a reversed NFA. Specifically, each state in Q3 corresponds to a subset of states in the powerset of Q₂. The initial state s₃ is {f}. Q₃ is initialized to {{f}}, and states r in Q₃ are explored by constructing for each aεΣ the following set:

P(r,a)={pεQ ₂:(p,a,q)εΔ₂ for some qεr},   Equation (3)

Equation (3) represents the set of states from which there is a transition labeled a to an element of r. If this set is not empty, it is added to Q₃ and the transition (r, a, P(r, a)) is added to Δ₃. Each state in Q₃ previously not explored is explored until there are no states in Q₃ left to explore. The set of final states in M₃, F₃, is the set of all states q in Q₃ such that q∩S₂ is not empty. As discussed above, FIG. 4B illustrates the automaton M₂ for the regular expression ((a)*|b)(ab|b). Further, FIG. 5 illustrates the automaton M₃ for the same regular expression, where v={16}, w={13, 14}, x={8}, y={5}, and z={5,12}.

M₂ and M₃ may be used to construct another automaton, M₄ by the M₄ automaton generation module 108. The automaton M₄ is a DFA except that the transition function is a four-tuple, i.e. Δ₄ ⊂Q₂ ^(x) _(Q3) x Q₂ x T*. The DFA is similar to M₂ with one extra state, where the input alphabet is Q₃ instead of Σ, and some edges are deleted. The description of automaton M₄ that follows will use some notation concerning Σ₁ and M₁. Let

be the lexicographic partial ordering on the set of strings in Σ*₁ generated by the relation {(−, +)}∪{(a,a):aεΣ₁} on Σ₁.

Next a new state labeled ‘0’ is introduced, which is the start state of M₄. To define Δ₄, let (q, P, p, τ) be in Δ₄ if there exist q, rεQ₂, PεQ₃, pεP, aεΣ, such that (q, a, r)εΔ₁, π(r, p) is defined, and

$\begin{matrix} {\tau = {{\pi \left( {r,p} \right)} = \left. \left( {\max\limits_{\prec}\left\{ {{l\; {s\left( {\lambda \left( {r,p^{\prime}} \right)} \right)}\text{:}p^{\prime}} \in P} \right\}} \right) \right|_{T}}} & {{Equation}\mspace{14mu} (4)} \end{matrix}$

Similarly, let (0, P, p, τ) be in Δ₄ if there exist PεQ₃, pεP such that π(1, p) is defined, and

$\begin{matrix} {\tau = {{\pi \left( {r,p} \right)} = \left. \left( {\max\limits_{\prec}\left\{ {{l\; {s\left( {\lambda \left( {r,p^{\prime}} \right)} \right)}\text{:}p^{\prime}} \in P} \right\}} \right) \right|_{T}}} & {{Equation}\mspace{14mu} (5)} \end{matrix}$

It can be proved that these maximal elements exist, and are unique.

FIG. 6 illustrates the relationships among q, r, P, p, and τ in the construction of the transition function for M₄. Referring to FIG. 6, for the construction of M₄, q, r, p, εQ₁, P={p₁, . . . , p_(n)}εQ₃, each path λ(r, p_(i)) has label sequence τ_(i), and τ=π(r, p)=(max

{τ₁, . . . , τ_(n)})|_(T). FIGS. 7A and 7B illustrate the automaton M₄ for the regular expression ((a)*|b)|(ab|b), showing the inputs from Q3 and the outputs in T*, respectively, and FIG. 7C illustrates a simplified diagram of an input string, automata M₃ and M₄, and boundaries.

As discussed above, if an input string matches the regular expression, the pattern created by the pattern creation module 102 may be used to extract submatches by the extraction module 104 as follows.

Extracting the submatches for a string a₁ . . . a_(τ)εΣ* may occur as follows. The extraction process is subdivided in steps 1-3 for facilitating the description of the submatch extraction system 100. First, for step 1, the string a_(l)a_(l−1) . . . a₁ is processed using M₃. As it is processed, the states q_(l), q_(l−1), . . . visited during the processing are journaled, where q_(l) is {f}, the initial state of M₃. If the processing terminates before the whole input string has been processed, or terminates with q₀∉₃, it is reported that the string does not match, and the submatch extraction terminates. It can be proved that if M₃ has been constructed in the way described in the example, this will happen if and only if the string does not match.

Next, for step 2, if the submatch extraction did not terminate in the previous step, the system 100 may run M₄ on input q₀, q₁, . . . q_(l), using an additional data structure along the way in order to discover the submatch values for each capturing group. The data structure may include an array of length 2 c, indexed by elements of T, all initialized to NULL. While processing the i^(th) transition, namely (q_(i), P, q_(i+1), εΔ₄, for each tag in τεT*, the system 100 may write i in the array entry corresponding to the tag, overwriting the current entry. It can be proved that if M₄ is constructed in the way described in this example, this process will not terminate before all of q₀, q_(q) ₁, . . . q_(l) have been processed by M₄.

Next, for step 3, the submatch extraction system 100 uses the resulting array to read off the submatches from the input string, as follows. If the array entries for the tags S_(j) and E_(j) are s_(j) and e_(j), respectively, then the system reports that the submatch for capturing group j is a_(sj+1) . . . a_(ej). If the array entries for S_(j) and E_(j) are NULL, then the system reports that there is no submatch for the j^(th) capturing group. It can be proved that if M₃ and M₄ are the automata described in the example, then in the case that the array entries for S_(j) and E_(j) are NULL, there is indeed no submatch. It can also be proved that if M₃ and M₄ are the automata described in this example, the assignment of submatches that is reported by the system is valid.

Referring to FIGS. 5 and 7A-7C, an example of processing an input string aaab for the regular expression ((a)*|b)|(ab|b) is described. As discussed above, FIG. 5 illustrates the automaton M₃ for the regular expression ((a)*|b)(ab|b), where the states in FIG. 5 correspond to sets of states in FIG. 4B as follows: v={16}, w={13, 14}, x={8}, y={5}, and z={5,12}. Referring to FIG. 5, in step 1, the reversed input string is processed as baaa using M₃. As shown in FIG. 5, the processing begins at v, then proceeds from v to w (processing the symbol b), then proceeds from w to z (processing a), then proceeds from z to y (processing another a), then remains at y (processing another a). The states journalled during this processing are v, w, z, y and y, i.e. states {16}, {13, 14}, {5, 12}, {5}, {5} respectively. These states are then input in reverse order into the automaton M₄. Referring to FIG. 7A, the states visited are {16}, {13, 14}, {5, 12}, {5}, {5}, and respectively correspond to v, w, z, y and y. For the processing of these states in this example, referring to FIGS. 7A and 7B the state of M₄ is initially 0, then transitions to 5 as a result of receiving the input {5}, giving output S₁S₂, then remains at 5 after receiving input {5}, giving output E₂S₂, then remains at 5 after receiving input {5, 12}, giving output E₂S₂, then transitions to 14 as a result of receiving input {13, 14}, giving output E₂E₁S₃, and transitions to 16 as a result of receiving input {16}, giving output E₃. Thus in step 2, the submatch extraction system 100 runs automaton M₄ with input ({5}, {5}, {5, 12}, {13, 14}, {16}), writing entries in the array with each transition. The resulting array reads as follows:

[S₁, E₁, S₂, E₂, S₃, E₃]=[0, 3, 2, 3, 3, 4]  Equation (7)

In step 3, the extraction module 104 reads from the array that the three capturing groups have respective submatches aaa, a, and b. For example, referring to FIG. 7C, it can be seen that the last instance of each S and each E is kept. The submatches to the capturing groups are read off from the input string aaab using this array, resulting in submatch aaa to capturing group 1, a to capturing group 2 and b to capturing group 3.

3. Method

FIGS. 8 and 9 illustrate flowcharts of methods 200 and 300 for submatch extraction, corresponding to the example of a submatch extraction system whose construction is described in detail above. The methods 200 and 300 may be implemented on the submatch extraction system with reference to FIGS. 1-7C by way of example and not limitation. The methods 200 and 300 may be practiced in other systems.

Referring to FIG. 8, at block 201, the example method includes receiving a regular expression.

At block 202, the example method includes converting the regular expression with capturing groups into a plurality of finite automata to extract submatches. In this example method, these are the automata M₁, M₂, M₃ and M₄ whose construction is described above.

At block 203, the example method includes receiving an input string.

At block 204, the example method includes using a first automaton (i.e., M₃) to determine whether the input string is in a language described by the regular expression, and to process the input string.

At block 205, the example method includes using states of the first automaton (i.e., M₃) in a second automaton (i.e., M₄) to extract the submatches. This includes reversing the input string and processing the reversed input string by the automaton M₃, and using the states visited during the processing of the reversed input string in reverse order as input to the automaton M₄ to extract the submatches. Indices may be assigned to the capturing groups for the regular expression, the start and end locations of each capturing group may be determined, and the start and end locations may be used to extract the submatches.

Referring to FIG. 9, the further detailed method 300 for submatch extraction is described. At block 301, the example method includes receiving a regular expression and an input string.

At block 302, the example method includes generating the finite automaton M₁, whose construction is described above.

At block 303, the example method includes converting the automaton M₁ into another automaton, the NFA M₂, whose construction is described above.

At block 304, the example method includes converting the automaton M₂ into the DFA M₃, whose construction is described above.

At block 305, the example method includes using M₁, M₂ and M₃ to construct another automaton, M₄, whose construction is described above. Thus, the example method includes converting the regular expression with capturing groups into the automata M₃ and M₄.

At block 306, the example method includes reversing the input string and processing the reversed input string by the automaton M₃, and using the states visited during the processing of the reversed input string in reverse order as input to the automaton M₄ to extract the submatches. The method also includes assigning indices to the capturing groups for the regular expression, determining start and end locations of each capturing group, and using the start and end locations to extract the submatches.

4. Computer Readable Medium

FIG. 10 shows a computer system 400 that may be used with the examples described herein. The computer system 400 represents a generic platform that includes components that may be in a server or another computer system. The computer system 400 may be used as a platform for the system 100. The computer system 400 may execute, by a processor or other hardware processing circuit, the methods, functions and other processes described herein. These methods, functions and other processes may be embodied as machine readable instructions stored on computer readable medium, which may be non-transitory, such as hardware storage devices (e.g., RAM (random access memory), ROM (read only memory), EPROM (erasable, programmable ROM), EEPROM (electrically erasable, programmable ROM), hard drives, and flash memory).

The computer system 400 includes a processor 402 that may implement or execute machine readable instructions performing some or all of the methods, functions and other processes described herein. Commands and data from the processor 402 are communicated over a communication bus 404. The computer system 400 also includes a main memory 406, such as a random access memory (RAM), where the machine readable instructions and data for the processor 402 may reside during runtime, and a secondary data storage 408, which may be non-volatile and stores machine readable instructions and data. The memory and data storage are examples of computer readable mediums. The memory 406 may include modules 420 including machine readable instructions residing in the memory 406 during runtime and executed by the processor 402. The modules 420 may include the modules 101-108 of the system 100 shown in FIG. 1.

The computer system 400 may include an I/O device 410, such as a keyboard, a mouse, a display, etc. The computer system 400 may include a network interface 412 for connecting to a network. Other known electronic components may be added or substituted in the computer system 400.

What has been described and illustrated herein is an example along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated. 

What is claimed is:
 1. A method for submatch extraction, the method comprising: receiving an input string; receiving a regular expression; and converting, by a processor, the regular expression with capturing groups into a plurality of finite automata to extract submatches, wherein the extracting includes: using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string; and using states of the first automaton in a second automaton to extract the submatches.
 2. The method of claim 1, further comprising: inputting a reversed input string into the first automaton; and inputting the states in reverse order into the second automaton.
 3. The method of claim 1, further comprising: implementing the method in a parser.
 4. The method of claim 1, further comprising: implementing the method in a security information and event management (SIEM) system.
 5. A submatch extraction system comprising: a memory storing a module comprising machine readable instructions to: receive an input string; receive a regular expression; and convert the regular expression into a plurality of finite automata to extract submatches, wherein the extracting includes: using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string; and using states of the first automaton in a second automaton to extract the submatches; and a processor to implement the module.
 6. The submatch extraction system of claim 5, wherein the machine readable instructions are to: convert the regular expression with capturing groups into the plurality of finite automata to extract the submatches.
 7. The submatch extraction system of claim 5, wherein the machine readable instructions are to: input a reversed input string into the first automaton; and input the states in reverse order into the second automaton.
 8. The submatch extraction system of claim 5, wherein the system is used in a parser.
 9. The submatch extraction system of claim 5, wherein the system is used in a security information and event management (SIEM) system.
 10. A non-transitory computer readable medium having stored thereon machine readable instructions for submatch extraction, the machine readable instructions when executed cause a computer system to: receive an input string; receive a regular expression; and convert, by a processor, the regular expression into a plurality of finite automata to extract submatches, wherein the extracting includes: using a first automaton to determine whether the input string is in a language described by the regular expression, and to process the input string; and using states of the first automaton in a second automaton to extract the submatches.
 11. The non-transitory computer readable medium of claim 10, the machine readable instructions when executed cause the computer system to: convert the regular expression with capturing groups into the plurality of finite automata to extract the submatches.
 12. The non-transitory computer readable medium of claim 10, the machine readable instructions when executed cause the computer system to: input a reversed input string into the first automaton; and input the states in reverse order into the second automaton. 